Newton's 3 Laws, with a bicycle - Joshua Manley
從自行車學牛頓三大運動定律 - 約書亞曼利 (Joshua Manley)
2,083,313 views ・ 2012-09-19
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譯者: Allen Li
審譯者: Jephian Lin
00:14
Have you ever noticed that it's harder
to start pedaling your bicycle
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你曾注意到嗎?
騎自行車時
00:17
than it is to ride at a constant speed?
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起步比等速前進費力
00:20
Or wondered what causes your bicycle to move?
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你想過自行車為什麼會移動嗎?
00:23
Or thought about why it goes forward
instead of backwards or sideways?
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為什麼是前進
而不是後退或向旁邊呢?
也許你沒想過,但不止是你
00:27
Perhaps not, and you wouldn't be alone.
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一直到十七世紀
00:30
It wasn't until the 17th century
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00:31
that Isaac Newton described
the fundamental laws of motion
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牛頓 (Isaac Newton) 描述了
三大運動定律
00:34
and we understood the answer
to these three questions.
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我們才明白了這三個問題的答案
00:37
What Newton recognized was that
things tend to keep on doing
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牛頓發現
事物傾向於保持原來狀態
00:40
what they are already doing.
So when your bicycle is stopped,
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所以,當你的自行車停止
00:43
it stays stopped, and when it is going,
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它會保持停止狀態;如果它前進
00:46
it stays going.
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則會保持前進狀態
00:48
Objects in motion tend to stay in motion
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動者恆動
00:50
and objects at rest tend to stay at rest.
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靜者恆靜
這就是牛頓第一運動定律
00:53
That's Newton's First Law.
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00:55
Physicists call it the Law of Inertia,
which is a fancy way of saying
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物理學家稱為慣性定律
這只是比較酷的名稱
00:59
that moving objects don't spontaneously
speed up, slow down, or change direction.
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意思是運動中的物體不會自己
加速、減速、改變方向
01:03
It is this inertia that you must overcome
to get your bicycle moving.
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必須克服慣性,自行車才會移動
01:09
Now you know that you have to overcome
inertia to get your bicycle moving,
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雖然知道克服慣性,自行車才會動
01:12
but what is it that allows you to overcome it?
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但克服慣性的到底是什麼?
01:14
Well, the answer is explained by Newton's Second Law.
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牛頓第二運動定律說出了答案
牛頓第二定律,用數學觀點來說
01:18
In mathematical terms, Newton's Second Law says
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01:20
that force is the product of mass times acceleration.
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力 = 質量 × 加速度
01:24
To cause an object to accelerate, or speed up,
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要讓某個物體加速
01:27
a force must be applied.
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必須施力
01:29
The more force you apply,
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施力越大
01:31
the quicker you accelerate.
And the more mass your bicycle has,
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加速越快。而且自行車質量越大
01:34
and the more mass you have too,
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還有你的質量越大
01:36
the more force you have to use
to accelerate at the same rate.
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要施更大的力,才有相同的加速度
01:39
This is why it would be really difficult
to pedal a 10,000 pound bicycle.
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如果自行車有一萬磅重
會很難踩得動
01:43
And it is this force, which is applied
by your legs pushing down on the pedals,
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而就是雙腿踩踏板的力量
讓你克服牛頓的慣性定律
01:49
that allows you to overcome Newton's Law of Inertia.
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你越用力踩踏板
01:52
The harder you push down on the pedals,
the bigger the force
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01:54
and the quicker you accelerate.
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加速就越快
01:56
Now on to the final question:
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最後一個問題是:
01:58
When you do get your bike moving,
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當你移動自行車時
02:00
why does it go forward?
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為什麼自行車會往前走?
根據牛頓第三定律,每個作用力
02:03
According to Newton's Third Law,
for every action,
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02:05
there is an equal and opposite reaction.
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都有大小相等、方向相反的
反作用力
02:07
To understand this, think about what
happens when you drop a bouncy ball.
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要理解這點,想想彈力球掉到地上
會怎麼樣
彈力球掉到地上時
02:12
As the bouncy ball hits the floor,
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02:13
it causes a downward force on the floor.
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對地板施了個向下的力
02:15
This is the action.
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這是作用力
02:17
The floor reacts by pushing
on the ball with the same force,
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地板的反應是,用相等的力量推球
02:21
but in the opposite direction, upward,
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但方向相反,變成向上
02:24
causing it to bounce back up to you.
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讓球彈回你手上
02:27
Together, the floor and the ball form what's called
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地板和球,形成了一對
02:29
the action/reaction pair.
When it comes to your bicycle,
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作用力與反作用力
02:32
it is a little more complicated.
As your bicycle wheels spin
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自行車則更複雜
02:35
clockwise, the parts of each tire
touching the ground
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當車輪順時針旋轉
輪子接觸地面部分
02:39
push backwards against the Earth:
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會向後推地球
02:41
the actions. The ground pushes
forward with the same force
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這是作用力。地面以相等的力量
02:45
against each of your tires: the reactions.
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反方向推輪子,這是反作用力
因為自行車有兩個輪子
每個會與地面形成一對
02:49
Since you have two bicycle tires,
each one forms an action/reaction pair
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02:53
with the ground. And since
the Earth is really, really, really big
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作用力與反作用力
因為和自行車相比
02:56
compared to your bicycle, it barely moves
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地球真的超級大,
所以你的自行車輪胎向後的力量
02:59
from the force caused by your bicycle
tires pushing backwards,
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幾乎沒有讓地球移動
03:02
but you are propelled forward.
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──然而你就被反作用力推向前了
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